Method and apparatus for performing optical coherence-domain reflectometry and imaging through a scattering medium employing a power-efficient interferometer

ABSTRACT

An apparatus and method for performing optical coherence domain reflectometry. The apparatus preferably includes a single output light source to illuminate a sample with a probe beam and to provide a reference beam. The reference beam is routed into a long arm of an interferometer by a polarizing beamsplitter. A reflected beam is collected from the sample. A 90° double pass polarization rotation element located between the light source and the sample renders the polarizations of the probe beam and reflected beam orthogonal. The polarizing beamsplitter routes the reflected beam into a short arm of the interferometer. The interferometer combines the reference beam and the reflected beam such that coherent interference occurs between the beams. The apparatus ensures that all of the reflected beam contributes to the interference, resulting in a high signal to noise ratio.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of copending U.S. patent application09/042,205 filed Mar. 13, 1998 (still pending), which is hereinincorporated by reference.

FIELD OF THE INVENTION

This invention relates generally to confocal microscopy and opticalcoherence domain reflectometry (OCDR). More specifically, it relates todevices for measuring optical reflectance and imaging through athickness of biological tissue.

BACKGROUND OF THE INVENTION

There are a number of applications for techniques for opticalmeasurement through light scattering materials. Most notably, suchmeasurements can be performed through biological tissues and thereforecan be used for noninvasive medical diagnostic tests. Cancer tissue andhealthy tissue, for example, can be distinguished by means of differentoptical properties. Scanning the optical measurement can yield highcontrast and high magnification images of biological tissues. Forexample, imaging techniques could be used to examine plaque on theinterior walls of arteries and vessels or other small biologicalstructures. Related applications extend to the examination andtroubleshooting of integrated optical circuits, fiber optic devices, andsemiconductor structures. All these applications require that themeasuring technique have a relatively high spatial resolution (micronsor tens of microns), high sensitivity, and low noise.

Optical time domain reflectometry (OTDR) and optical frequency domainreflectometry (OFDR) are techniques which are used to examine opticalsystems and are generally not capable of performing high resolutionmeasurements through a light scattering material. For example, thesemethods are generally designed for finding and locating (to within 1meter) flaws in a fiber optic system.

Optical coherence domain reflectometry (OCDR) is a technique which hasbeen used to image an object within or behind light scattering media.The technique uses short coherence length light (typically with acoherence length of about 10-100 microns) to illuminate the object.Light reflected from a region of interest within the object is combinedwith a coherent reference beam. Interference occurs between the twobeams only when the reference beam and reflected beam have traveled thesame distance. This allows the OCDR to discriminate against lightscattered from outside the region of interest.

FIG. 1 shows a typical OCDR setup similar to ones disclosed in severalU.S. patents (U.S. Pat. Nos. 5,465,147, 5,459,570, and 5,321,501 issuedto Swanson et al., U.S. Pat. Nos. 5,291,267, 5,365,335, and 5,202,745issued to Sorin et al). FIG. 1 shows the device made with fiber opticcomponents, but OCDR devices can also be made with bulk opticalcomponents. Light having a short coherence length l_(c) (given byl_(c)=C/Δf, where Δf is the spectral bandwidth) is produced by a lightsource 20 and travels through a 50/50 coupler 22 where it is dividedinto two paths.

One path goes to the sample 24 to be analyzed and the other path goes toa movable reference mirror 26. Extra fiber length in the reference pathis shown as fiber loop 31. The probe beam reflected from the sample 24and reference beam reflected from the reference mirror 26 are combinedat the coupler 22 and sent to a detector 28. The optical paths traversedby the reflected probe beam and reference beam are matched to within onecoherence length such that coherent interference can occur uponrecombination at the coupler.

A phase modulator 30 (such as a piezoelectric fiber stretcher) producessideband frequencies in the probe beam which produce a temporalinterference pattern (beats) when recombined with the reference beam.The detector 28 measures the amplitude of the beats. The amplitude ofthe detected interference signal is a measure of the amount of lightscattered from within a coherence gate interval 32 inside the sample 24that provides equal path lengths for the probe and reference beams.Interference is produced only for light scattered from the sample 24which has traveled the same distance (to within approximately onecoherence length) as light reflected from the mirror 26. The coherencegate interval 32 has a width of approximately one coherence length. Thisfeature of OCDR allows the apparatus to discriminate against light whichis scattered from outside the coherence gate interval 32, and which isusually incoherent compared to the reference beam. This discrimination(a ‘coherence gate’) results in improved sensitivity of the device.

One negative consequence of the geometry of FIG. 1 is that 50% of thelight reflected from the sample 24 is lost. On its return trip throughthe coupler 22, half the reflected probe beam enters the light source 20and does not enter the detector 28. This is undesired because itdecreases the signal to noise ratio of the device and results in a morepowerful light source being required. Another negative feature of thedevice of FIG. 1 is that it requires the use of a moving mirror to scanlongitudinally in and out of the sample 24. The use of a movingmechanical mirror is a disadvantage because moving mechanical partsoften have alignment and reliability problems.

Another disadvantage of the device of FIG. 1 is the requirement for alarge depth of focus of the probe beam in sample 24. A large depth offocus is necessary to allow longitudinal scanning of the coherence gateinterval 32 while maintaining the coherence gate interval in the regionof the beam having a reasonably small spot size. This requirementincreases the minimum spot size of the beam, and thus limits the spatialresolution of the device when acquiring images.

A further disadvantage of the device of FIG. 1 is the long integrationtime typically necessary for each measurement point (pixel) whenacquiring an image. This is due to the low power of the backreflectedsignal when imaging deep within a scattering medium. Under theseconditions, the slow acquisition time does not allow in-vivo imaging oflive tissue which is usually in motion.

U.S. Pat. No. 5,291,267 to Sorin et al. discloses a technique for OCDRwhich uses the light source as a light amplifier in order to boost thereflected signal from the sample. Light reflected from the sample isreturned through the light source in a reverse direction and isamplified as it passes through. However, Sorin's device requires acoupler in the light path between the source and sample and sonecessarily wastes 50% of the light reflected from the sample. In otherwords, only 50% of the light reflected by the sample is amplified andcontributes to the interference signal. Consequently, Sorin's deviceproduces less than optimum signal to noise ratio resulting in lessaccurate measurements.

OBJECTS AND ADVANTAGES OF TE INVENTION

Accordingly, it is a primary object of the present invention to providea method and apparatus for reflectance measurements through lightscattering materials that:

1) requires fewer active components than prior art techniques;

2) utilizes all the light reflected from the sample;

3) provides an improved signal to noise ratio compared to prior arttechniques;

4) is able to scan deep into scattering materials such as biologicaltissue;

5) is able to produce vertical-section, horizontal-section, or threedimensional images;

6) has short data acquisition times for real-time in-vivo imaging oflive tissue;

7) has high spatial resolution thus enabling the imaging of microscopicstructures such as biological structures; and

8) is easy and inexpensive to manufacture.

These and other objects and advantages will be apparent uponconsideration of the following description and accompanying drawings.

SUMMARY

The invention presents a method and apparatus for performing opticalcoherence domain reflectometry on a sample of light scattering media. Ina preferred embodiment, the apparatus includes a single output lightsource for illuminating a sample, a polarizing beamsplitter, a 90°double pass polarization rotation element, and an interferometer havinga short arm and a long arm. The sample is illuminated with a probe beamemitted from the light source. The probe beam travels to the samplealong a light path. The polarizing beamsplitter is positioned to splitoff a reference beam from the probe beam and to route the reference beaminto the long arm of the interferometer.

The probe beam reflects from the sample, producing a reflected beam. Thepolarization rotation element is located between the light source andsample such that the reflected beam and the probe beam have orthogonalpolarizations. The reflected beam is reflected from the polarizingbeamsplitter and routed into the short arm of the interferometer. Thearms of the interferometer have optical path lengths selected to providecoherence between the reference beam and the reflected beam.

According to an alternative embodiment, the apparatus includes a singleoutput light source for illuminating the sample, a first beamsplitter, asecond polarizing beamsplitter, a 90° double pass polarization rotationelement, and an interferometer having a short arm and a long arm. Thesample is illuminated with a probe beam emitted from the light source.The probe beam travels to the sample along a light path. The firstbeamsplitter splits off a reference beam from the probe beam and routesthe reference beam into the long arm of the interferometer.

The probe beam reflects from the sample, producing a reflected beam. Thepolarization rotation element is located between the light source andthe sample such that the reflected beam and the probe beam haveorthogonal polarizations. The second polarizing beamsplitter is locatedsuch that the reflected beam is separated from the probe beam and routedinto the short arm of the interferometer. The reference beam and thereflected beam are combined at the output of the interferometer, wherecoherent interference occurs. The lengths of the long arm and short armare selected to provide coherence between the reference beam and thereflected beam.

The first beamsplitter is preferably a polarizing beamsplitter so thatthe probe beam and the reference beam have orthogonal polarizations. Thelight source can also be polarized. In this case, if the firstbeamsplitter is a polarizing beamsplitter, then the amount of opticalpower routed into the reference beam can be controlled by adjusting theorientation of the first beamsplitter with respect to the polarizationof the light source.

The interference is modulated by modulating the phase or frequency ofthe reference beam or the probe beam with a phase modulator. The phasemodulator can be located within one arm of the interferometer or in thelight path. Preferably, the phase modulator is driven sinusoidally. Onearm of the interferometer can include a variable optical attenuator tocontrol the amount of optical power in the reference beam. The lightpath between the light source and the sample can comprise an opticalfiber. The light source preferably has a coherence length of less than3000 microns. The light source also preferably produces light in thewavelength range of 0.8 to 1.6 microns, over which range biologicaltissues are particularly transparent.

The light path preferably includes a transverse scanning mechanism forscanning the probe beam within the sample. Such a scanning mechanism canhave a micromachined scanning mirror. A longitudinal scanning mechanismcan also be provided to scan in a direction parallel to the probe beam.Scanning allows the apparatus to create images. Longitudinal scanning inthe direction of the probe beam axis, along with scanning in a directionperpendicular to the axis, provides the possibility of obtaining animage of a vertical cross section of the sample.

The light path can also have a lens for focusing the probe beam to asmall spot within the sample. The lens preferably has a numericalaperture in the range of 0.4 to 1.4. The interferometer is preferably aMach-Zehnder type made of two unequal lengths of optical fiber coupledon each end by fiber optic couplers.

In an alternative embodiment, the apparatus includes a two output lightsource, a phase modulator, an interferometer having unequal path lengths(arms), and an optical detector. The light source emits a probe beamfrom a probe aperture and a reference beam from a reference aperture.The interferometer is disposed in optical communication with thereference aperture so that the reference beam travels directly into theinterferometer.

The probe beam travels to the sample through the phase modulator whichis located between the light source and the sample. The probe beamreflects from the sample, producing a reflected beam. The reflected beamtravels back to the light source from the sample. The light source isalso preferably an amplifier and so amplifies the reflected beam. Thereflected beam and reference beam are combined inside the light sourceand pass into the interferometer. The light source is located such thatthe reflected beam is collinear with the reference beam at the referenceaperture.

The interferometer has a beamsplitter for splitting the reference beamand reflected beam into arms of unequal lengths. The lengths of theinterferometer arms are selected to restore the optical coherencebetween the reference beam and the reflected beam. Preferably, one armof the interferometer has a variable optical path length device foradjusting the path length difference between the arms. The phasemodulator is driven by an oscillator such that the reference beam andreflected beam have different spectral characteristics. Therefore, thereference beam and the reflected beam produce a temporal interferencepattern at the output of the interferometer. The optical detector ispositioned in optical communication with the interferometer to detectthe temporal interference.

The apparatus can also have a 90° double pass polarization rotationelement located in the light path such that the reflected beam has apolarization orthogonal to the reference beam. This allows the referencebeam and reflected beam to be separated into the different arms of theinterferometer. One arm of the interferometer can have a 90°polarization rotator which renders the polarizations of the referenceand reflected probe beams parallel before being recombined.

In the case where the light path has a 90° polarization rotationelement, the interferometer can be a length of birefringent opticalfiber oriented such that the reference beam experiences a higher indexof refraction than the reflected beam. Further, if the birefringentoptical fiber has a non-zero coefficient of birefringence, then theoptical path length difference between the interferometer arms can becontrolled by controlling the temperature of the birefringent fiber.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a prior art optical coherence domain reflectometer.

FIG. 2 shows an optical coherence domain reflectometer made with bulkoptical components according to a first embodiment of the presentinvention.

FIG. 3 is a close-up view of a focal point within a sample medium.

FIG. 4 is a close-up view of a focal point of a prior art apparatus.

FIG. 5A shows a spatial filter which can be used in conjunction with areflectometer according to the present invention.

FIG. 5B shows a graph which illustrates the signal response from acoherence gate and from a confocal microscope.

FIG. 6 is a reflectometer according to the present invention which usesfiber optic components.

FIGS. 7A-7D show various examples of fiber-coupled two-output lightsources which can be used in the reflectometer of FIG. 6.

FIGS. 8A-8B show close-up views of transverse scanning heads used toprovide imaging capability.

FIG. 8C shows an example of a vertical scanning head which can be usedto scan in a direction parallel to a probe beam.

FIGS. 9A-9B show two frequency domain graphs illustrating the effectthat a phase modulator has upon the spectrum of the probe beam.

FIG. 10 shows a reflectometer according to the present invention whichuses polarized light.

FIG. 11 shows a reflectometer according to the present invention whichuses polarized light and a birefringent fiber.

FIG. 12 shows an optical mixer which can be used in an interferometer tocoherently combine two beams with orthogonal polarizations.

FIG. 13 shows a reflectometer according to another embodiment of thepresent invention which uses polarized light.

FIG. 14 shows a reflectometer according to an alternative embodiment ofthe present invention which uses a single output light source.

FIG. 15 shows a reflectometer according to another embodiment of thepresent invention.

FIG. 16 shows a reflectometer according to a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION

FIG. 2 shows a first embodiment of an apparatus for performing opticalcoherence domain reflectometry on a sample of light scattering media. Inthe first embodiment, the apparatus is constructed from bulk opticalcomponents. A two output light source 40 illuminates a sample 52 with aprobe beam 44. The sample comprises a light scattering medium such asbiological tissue. The light source emits the probe beam 44 from a probeaperture 48. The light source also emits a reference beam 42 from areference aperture 49. The reference beam 42 is emitted directly into aninterferometer having a short arm 64A and a long arm 64B. Theinterferometer shown is a Mach-Zehnder type, but other types can be usedas well. The interferometer outputs light into two optical detectors 70which detect interference. The detectors output to a signal processor 72and a computer 74.

The light source 40 preferably has a relatively short coherence lengthin the range of 30 to 3000 microns. The reference beam 42 and the probebeam 44 are counterpropagating. Proper choice of a light source for agiven application involves selecting a proper coherence length andwavelength which optimizes the sensitivity of the device. The referencebeam 42 and the probe beam 44 are coherent with respect to one anotherand travel in opposite directions. The light source 40 of the embodimentof FIG. 2 should be a two output light source to provide thecounterpropagating reference beam and probe beam. The light source 40may be, for example, a semiconductor optical amplifier or fiber opticalamplifier which emits light from front and rear apertures. In caseswhere higher probe beam power is desired, the light source 40 may be asemiconductor laser or fiber laser which emits from front and rearcavity reflectors.

The probe beam 44 exits the probe aperture 48 and travels along a lightpath 50 to the sample 52. The light path 50 passes through a phasemodulator 54 which varies the phase of the probe beam 44 at apredetermined frequency F₁. The phase modulator is electrically drivenby an oscillator 56. Preferably, the phase of the probe beam 44 isvaried in a sinusoidal fashion, although other modulation waveforms canbe used. The phase modulator can be a piezoelectric fiber stretcher (inthe case where optical fiber components are used), or electroopticcrystal, for example. Other methods well known in the art of lightmodulation can also be used. A lens assembly 58 focuses the probe beam44 to a focal point 60 within the sample 52.

A portion of the light scattered or reflected by the sample 52 at thefocal point 60 is collected by the lens 58 and travels in a directionexactly opposite the original probe beam 44. This reflected lightcomprises a reflected probe beam 62. The reflected probe beam 62 thustravels back through the phase modulator 54 and through the lightsource/amplifier 40. The reflected probe beam 62 is coherently amplifiedas it passes through the light source/amplifier 40. In this way, thelight source/amplifier performs two functions: light source and lightamplification. The amplified, reflected probe beam emerges from thereference aperture 49 of the light source/amplifier traveling collinearand parallel with the reference beam 42.

The two beams then enter the interferometer having two arms 64A, 64B ofdifferent path lengths. In the particular embodiment of FIG. 2, theinterferometer is comprised of 50/50 beamsplitters 65A, 65B and mirrors67. The path length difference between the two optical paths in theinterferometer is equal to twice the length ΔL. Length ΔL is equal tothe distance between beamsplitter 65A and mirror 67. When the reflectedprobe beam 62 arrives at the reference aperture 49, it has traveledfurther than the reference beam by a distance equal to twice the lengthL_(probe). It is noted that length L_(probe) is measured from the center41 of the light source 40 to the focal point 60 inside the sample 52.The apparatus is designed such that 2ΔL is equal to 2L_(probe) to withinapproximately one optical coherence length of the light source 40.Expressed mathematically, this relationship is:

|2ΔL−2L_(probe)|<L_(c),

where L_(c) is the coherence length of the probe and reference beams.Coherent interference between the reference beam 42 and reflected probebeam 62 occurs at beamsplitter 65B when this condition is satisfied. Theportion of the reference beam 42 which travels through the long arm 64Bof the interferometer interferes with the portion of the reflected probebeam 62 which travels through the short arm 64A of the interferometer.

The output of the interferometer is monitored by the two opticaldetectors 70. The detectors measure the light intensity and arepreferably high-speed photodiodes such as PIN photodiodes or avalanchephotodiodes. The signal received by the detectors 70 is a temporalfluctuation in light intensity (a temporal interference pattern) whosefrequency is determined by the driving frequency F₁ of the phasemodulator 54. For example, if the phase modulator 54 is drivensinusoidally at frequency F₁, then the detectors 70 will output a signalat frequency F₁. The amplitude of the detector signal at F₁ isdetermined by the amount of light reflected at the focal point 60.Therefore, the detector output amplitude at F₁ is a measure of thereflectance of the sample 52 at the focal point 60.

High-pass filters located within the signal processing unit 72 may benecessary to accurately measure the amplitude of F₁. The signalprocessor may also perform analog to digital conversion and output tothe computer 74, where reflectance measurements are stored. If the focalpoint 60 is scanned throughout the sample 52, then two dimensional orthree dimensional images of the reflectance of the interior of thesample 52 can be created from the time-varying F₁ signal. These imagescan be used to diagnose cancer or other internal medical conditions.

If the optical path between the probe aperture 48 and lens assembly isfixed, then at least one arm of the interferometer should have avariable optical path length to provide a mechanism for adjusting lengthΔL. For example, a variable optical path length can be provided bymoving mirrors 67, or by inserting a variable optical delay device intoone of the arms of the interferometer. The variable optical path lengthcan then be adjusted until coherent interference occurs at thebeamsplitter 65B and detectors 70.

Preferably, the phase modulator 54 is located in the light path 50between the light source 40 and sample 52 to modulate the probe beam 44.However, the reflected probe beam 62 can also be modulated by locatingthe phase modulator 54 in the short arm 64A of the interferometer. Phasemodulation of the reference beam 42 will also modulate the temporalinterference. This can be accomplished by locating the phase modulator54 in the long arm 64B of the interferometer. Therefore, the presentinvention can have the phase modulator 54 located within the light path50 or within either arm 64A, 64B of the interferometer.

It is noted that the portion of the reference beam 42 which travelsthrough the short arm 64A does not interfere with the portion of thereflected probe beam 62 which travels through the long arm 64B. Thisnon-interfering light does not contribute to the temporal interferencesignal and thus is noise in the system. Therefore, locating the phasemodulator 54 in either interferometer arm 64A, 64B may decrease thesignal-to-noise ratio of the detector signal.

The signal-to-noise ratio of the signal output by detector 70 can bemaximized by including an optical attenuator 76 in the long arm 64B (thereference path) of the interferometer. This is because the referencebeam 42 will (for most embodiments) be much brighter than the reflectedprobe beam 62. The reference beam may contain a component of noise knownas relative intensity noise (RIN) produced by the light source. Thenoise due to RIN is proportional to the signal strength. An alternativemethod of minimizing the effects of noise is the method of balanceddetection. Reference can be made to Takada et al., Applied PhysicsLetters, 59, page 143, 1991.

A major advantage of the present invention over the use of a standardconfocal microscope is that the signal to noise ratio of the signaloutput by detector 70 can be maximized by using as strong a referencesignal as possible. If the reflected probe beam is of amplitude A, andthe reference beam of amplitude B, the detected signal will beproportional to:

|(A+B)|² =|A| ²+2|AB|+|B| ².

Since one of the beams is phase modulated the only signal of frequencyF1 is the 2AB term. In the case of deep tissue imaging, then |A|<<|B|.Thus, the detected output is proportional to the amplitude of thereference signal as well as that of the reflected probe, and it isadvantageous to use as strong a reference signal as possible. Thisresult should be contrasted with that for the standard confocalmicroscope, where the detected output of the relatively weak reflectedprobe beam is proportional to |A|². Ideally, an input probe beam ofabout the same amplitude as the input reference beam is used so as toutilize the available power as efficiently as possible. An adjustableoptical attenuator 76 may be included in the long arm 64B (the referencepath) to avoid saturation of the detector or to attenuate the referencebeam if the light source is noisy.

FIG. 3 shows a close-up view of the focal point 60 within the sample.The coherence length of the light source 40 and the length of the longarm 64B of the interferometer establish a coherence gate interval 78located about the focal point 60. Probe beam light reflects from withinthe coherence gate interval 78 and travels the same distance (to withina coherence length) as the reference beam light 42 upon arrival at thebeamsplitter 65B and detectors 70. Therefore, the coherence gateinterval 78 can be moved back and forth in the probe beam path bychanging slightly the length of the long arm 64B or the short arm 64A.

The width of the coherence gate interval 78 can be increased (ordecreased) by increasing (or decreasing) the coherence length of thelight source 40. Preferably, the coherence gate interval 78 is centeredabout the focal point 60 and is fixed with respect to the focal point.Longitudinal scanning (in and out of the sample 52) of the focal point60 and coherence gate interval 78 is achieved by causing relative motionbetween the lens assembly 58 and the sample 52. In this way the focalpoint 60 always lies within the coherence gate interval 78 duringscanning of the focal point both in transverse and longitudinaldirections. Preferably, the coherence gate interval 78 is not used todefine the longitudinal image resolution, as is done in the cited priorart devices. Also, unlike in prior art devices, the apparatus does notscan the coherence gate interval through a fixed focal point having arelatively large depth of field to accomplish longitudinal scanning ofthe image. Instead, the apparatus uses the coherence gate only forreducing the “glare” or scattered photon noise coming from regions nearthe sample surface. This, in combination with using a high numericalaperture focusing lens, improves image contrast and allows a highertransverse spatial resolution than is obtained in the prior art devices.

The longitudinal and transverse resolution are determined by the depthof focus and spot diameter, respectively, of the focal point 60. Using alens assembly 58 having a high numerical aperture results in a smallspot diameter and shorter depth of focus, thereby increasing both thelongitudinal and transverse spatial resolutions. Preferably, the lensassembly 58 has a numerical aperture in the range of 0.4 to 1.4. In thecase of a high numerical aperture lens assembly, a short coherencelength laser used as the light source could provide a coherence gatewidth sufficient to improve the signal-to-noise ratio of the deviceprovided that the coherence length is less than the imaging depth. Forexample, in order to gain an improvement in signal-to-noise performance,the coherence length does not have to be 10 to 30 microns as used forz-scanning in prior art optical coherence detection imaging systemsemploying low numerical aperture focusing lenses. Instead, high powerand inexpensive diode lasers having coherence lengths of about 100 to1000 microns may be used. This increases the signal-to-noise ratio whilereducing cost.

Referring again to FIG. 2, the signal processor 72 and computer 74convert the amplitude of the interference signal produced by thedetectors 70 into a reflectance value for the location of the focalpoint 60. The focal point 60 and coherence gate interval (which arepreferably fixed with respect to one another) can be scanned throughoutthe sample 52 to produce a three dimensional image of the reflectance ofthe sample interior. Therefore, although the apparatus of FIG. 2 onlymeasures the reflectance at the focal point 60, the focal point 60 canbe scanned to produce three dimensional images, or two dimensionalimages in any plane. For example, vertical-section or horizontal sectionimages can be obtained.

FIG. 4 shows a close-up of the focal point 60 of a typical prior artOCDR device. Here, the coherence gate interval 78 is short compared tothe ‘length’ of the focal point. The long focal point is a result ofusing a low numerical aperture lens assembly. Scanning parallel with theprobe beam is performed by moving longitudinally the coherence gateinterval 78 while the focal point 60 remains fixed. This is done bymoving the scanning reference mirror 26 of FIG. 1. It is noted that thelow numerical aperture of the lens assembly used in the prior artembodiment of FIG. 4 results in a lower transverse spatial resolutiondue to larger focus spot size (i.e. larger focus ‘waist’).

Preferably, the light path 50 between the light source 40 and the sample52 includes a confocal microscopy apparatus or, equivalently, a spatialfrequency filter. FIG. 5A shows such an apparatus, which is well knownin the art of optics. The probe beam 44 and reflected probe beam 62 arefocused by lenses 58A, 58B and pass through a small aperture 80. Theaperture 80 helps to further discriminate against probe beam light whichdid not reflect from the focal point 60. This improves the accuracy ofthe reflectivity measurement of the focal point 60. It is known in theart of confocal microscopy that the end of an optical fiber can functionas the aperture of a spatial filter.

The advantage of using a high numerical-aperture lens 58B in theconfocal lens assembly of FIG. 5A is that it can provide a very shortrange definition. For example, this range definition can be about 5microns when lens 58B has a numerical aperture greater than 1 at awavelength of 1.3 microns. This makes it possible to obtain good imagesof a vertical cross section of tissue. However, if a confocal microscopewere used alone, the glare from tissue nearer to the lens 58B than thefocus would tend to give a response much like that shown as the confocalmicroscope response in FIG. 5B. This problem can be obviated bycombining the confocal microscope response and coherence gate response.

In the example shown in FIG. 5B, the response of the coherence gate iswider than the confocal microscope response above the −33 dB levelindicated at 59. The response obtained from a combinedconfocal/coherence gate response will be mainly from a region withinabout 60 microns from the focus. Outside of this range, the responsefollows that of the coherence gate, and falls off far more rapidly thanthe confocal microscope response. In this case, although the coherencegate might be of the order of 100 microns wide, the range definition ofthe high numerical-aperture lens can still be obtained.

The reflectometer of the first embodiment is able to measure deep intothe sample 52 because the optical system uses both a confocal microscopeassembly and coherence gating, which provides high signal-to-noiseperformance. Even if the coherence length is as long as a fewmillimeters, the system is far more sensitive than a standard microscopebecause it obtains an output signal linearly proportional to theamplitude A of the weak reflected probe beam rather than the square ofthe amplitude, A².

The reflectometer also ensures that nearly all the probe beam 44 (minussmall optical losses) reaches the sample 52, and nearly all thereflected probe beam 62 returns to the light source 40 to be amplified.This is because there is no optical splitting component such as acoupler between the light source 40 and the sample 52, as there are inthe prior art devices mentioned. Due to this arrangement, the lightsource 40 can function simultaneously as a light source and amplifier toamplify the weak reflected probe beam 62. This boosts the strength ofthe reflected probe beam which leads to deeper scanning capability orfaster scanning. Further, these benefits are provided without addingadditional active components.

FIG. 6 shows a second embodiment of the apparatus which differs from thefirst embodiment in that it uses optical fiber components. Optical fibercomponents are preferred for their low cost, ruggedness, flexibility,and resistance to misalignment. All of the optical fibers used in theembodiment of FIG. 6 are preferably single mode fibers. The light sourceis preferably an unpolarized light source. The light path 50 and theshort arm 64A and long arm 64B comprise single mode optical fibers.50/50 directional couplers 82A, 82B are substituted for thebeamsplitters 65 of FIG. 2. The couplers 82A, 82B may be, for example,evanescent wave couplers or fused fiber couplers.

The probe beam 44 exits the light source 40 and propagates through anoptical fiber to the sample. The phase modulator 54 in this embodimentcan be a piezoelectric fiber stretcher. As the probe beam 44 exits theend of the optical fiber, it passes through the lens assembly 58 whichfocuses the probe beam 44. The end 84 of the optical fiber acts as anaperture which, in combination with the lens 58, forms a spatialfilter/confocal microscope apparatus for the reflected probe beam 62.Therefore, when an optical fiber is used as the light path 50, aseparate spatial filter setup (as shown in FIG. 5) is not necessary. Thereflected probe beam 62 is amplified as it returns through light source40 as in the embodiment of FIG. 2. The amplified reflected probe beamand reference beam emerge from the light source 40 and enter into thelong arm 64B and short arm 64A. The outputs of the second coupler 82Bfeed into the two detectors 70. Of course, the phase modulator 54 canalso be located in either arm 64A, 64B of the interferometer, asdescribed above for the embodiment of FIG. 2.

FIGS. 7A, 7B, 7C, and 7D show examples of a few fiber-coupled two-portlaser light sources which can provide the counterpropagating referenceand probe beams 42, 44 used in the above embodiments of the presentinvention. The reference aperture 49 and the probe aperture 48 are shownfor each light source. All the laser light sources have resonantcavities and provide higher power output than simple optical amplifiers.

FIG. 7A shows a doped fiber laser which is pumped by a pump laserthrough a wavelength division multiplexer fiber coupler 83. The fiberlaser may have broadband Bragg reflectors 86 on each side. FIG. 7B showsa two port fiber coupled semiconductor laser. Lenses 88 on each side ofthe laser couple the laser to optical fibers. The Bragg reflectors 86 ofFIGS. 7A and 7B may be omitted to provide a weaker light source withshorter coherence length and which can provide amplification of thereflected probe beam. FIG. 7C shows a fiber ring laser coupled to afiber 90 with a coupler and pumped with a pump laser through anothercoupler. The output coupler 92 in this embodiment may be a 90/10 orsimilar low coupling tap coupler. FIG. 7D shows another light sourcesimilar to FIG. 7C in which the ring laser is replaced with a linearfiber laser 91 having Bragg reflectors 86 on each end. In theembodiments of FIGS. 7C and 7D, the reference aperture 49 and probeaperture 48 are located on the output coupler 92.

Typically, short coherence length light sources have lower (diffractionlimited) output power compared to long coherence length light sources.Therefore, a thin coherence gate interval will generally be associatedwith lower probe beam power. Depending on the scattering coefficient ofthe sample and the depth of imaging, a proper balance between thecoherence gate interval and probe beam power can be found. In some casesif the tissue depth of interest is less than, for instance, 500 microns,a laser with a coherence length of 500 microns or more may be used, andincreased sensitivity may be obtained by interfering the probe beam withthe reference beam. In other cases, in order to eliminate glare, it maybe necessary to work with a much shorter coherence length.

FIG. 8A shows a close-up of a scanning optical head which is preferablyused in the present invention. The scanning head is used to producereflectance images by moving the focal point 60 through the sample 52.The head is located at the sample end of the light path and sointerfaces the probe beam 44 into the sample and collects the reflectedprobe beam 62. The head comprises all the components within the dottedarea 94. The light path is an optical fiber 96. The optical fiberdelivers the probe beam 44 to a pair of mirrors 98A, 98B and the lensassembly 58. The second mirror 98B is preferably a silicon micromachinedscanning mirror and can be pivoted at high speed about 1 or 2 axes toprovide a line-scan or two-dimensional scanning of a plane 100 at apredetermined depth 102 within the sample. The depth 102 of the plane100 can be changed by moving the scanning head in a directionperpendicular to the surface of the sample 52. The coherence gateinterval 78 remains in a fixed position about focal point 60 as thescanning head is moved towards or away from sample 52 and as mirror 98Bpivots. In this way, images of human skin can be obtained which consistof vertical sections, much like the images that pathologists are alreadyaccustomed to viewing under a traditional microscope.

FIG. 8B shows the optical scanning head of FIG. 5A with a GRIN lens 103which has a flat bottom 104. The flat bottom 104 allows close contact tobe made between the lens 103 and the sample 52. An index matching fluidlayer between the lens 103 and sample 52 can be used to minimizereflections. The total size of the scanning head shown in FIGS. 8A and8B may be a millimeter or two on a side. The scanned area may be about100 spot diameters across. For example, a device having a focused spotsize of 5 μm may have a field of view of about 500 μm. A suitablemicromachined mirror assembly is described in Optics Letters,“Micromachined Scanning Confocal Microscope”, by D. L. Dickensheets andG. S. Kino, Vol. 21, No. 10, May 15, 1996, pp. 764-766. It will beapparent to one skilled in the art of optics that other scanning devicesmay be used.

FIG. 8C shows an example of a scanning head capable of vertical sectionscanning. A voice coil motor comprising magnets 202 and voice coils 200is used to move the focal point 60 in the Z-direction with respect tothe sample 52. The voice coils 200 are attached to a movable carriage204 and the magnets 202 are held fixed with respect to the sample 52.The lens assembly 58 and scanning mirror 98B are mounted to the carriage204. As the voice coil is activated, the lens assembly, scanning mirror,and the focal point 60 are moved in the Z-direction. Thus, opticalcoherence is maintained between the reference beam and reflected probebeam for all positions of the focal point 60.

A region inside the carriage 204, between the lens assembly 58 and thesample 52, is filled with a liquid 206. The refractive index of theliquid 206 can be selected to match the refractive index of the sample52, thereby minimizing reflections. The portion of the optical fiber 96within the liquid 206 flexes as the carriage 204 moves so the opticalpath length to the focal point 60 is the same for all positions of thefocal point 60. An O-ring seal 208 is provided to seal the liquid insidethe carriage 204. Vertical scanning is performed at a slow frame rate ofabout 30 Hz while faster horizontal scanning at about 10 KHz is providedby the scanning mirror 98B. This produces an image of a vertical section205 of the sample 52. Vertical section images can be used by thoseskilled in the art of performing biopsies. It will be apparent to oneskilled in the art that other actuators may also be used to createscanned images.

For applications involving imaging through human skin, wavelengths inthe range of 1.0 to 1.5 microns typically allow imaging to depths of nomore than 3,000 microns. Therefore, a coherence gate interval thickerthan 3,000 microns would not help reduce the unwanted backscatteredlight from the sample. Therefore, a light source having a coherencelength greater than about 3000 microns would not be particularly useful.The depth of imaging should be considered when choosing a light sourcewith a given coherence length. Also, it is noted that very shortcoherence lengths will require more accurate adjustment of the lengthsof the interferometer arms 64A, 64B. Light sources with very shortcoherence lengths are typically much lower power and more expensive thanlong coherence length laser diodes.

FIG. 9A shows the frequency spectrum of the original probe beam 44 andreference beam 42. FIG. 9B shows an example of the frequency spectrum ofthe probe beam 44 and reflected probe beam 62 after passing through thephase modulator 54. Driving the phase modulator sinusoidally atfrequency F₁ produces two sidebands 106 on either side of the originalprobe beam 44 in the frequency spectrum. These sidebands are separatedin frequency from the original probe beam 44 by frequency F₁. Therefore,when the (modulated) reflected probe beam is coherently recombined withthe (unmodulated) reference beam, a beat frequency at frequency F₁ iscreated.

If scanning is being performed by the optical head (as shown in FIGS.8A, 8B, and 8C) to create an image, then the beat frequency (F₁) shouldbe selected with the scanning rate in mind. More specifically, the beatfrequency should be substantially higher than the rate at which pixelsinside the scattering sample medium are measured. This assures that thesignal processor 72 is provided with several cycles over which it canmeasure the interference magnitude, and therefore the reflectance at thefocal point 60 corresponding to the pixel. For example, if pixels aremeasured at a rate of 1 Mhz (fast enough for video), then the beatfrequency (and the phase modulator driving frequency) should be at leastabout 10 Mhz. This will provide 10 interference fringes per pixel, whichis sufficient to measure fairly accurately the reflectance of eachpixel. The pixel scanning speed and the beat frequency may depend uponthe particular application, of course. If high accuracy is required ofthe reflectance measurements, then frequency F₁ may be increased or thepixel scanning rate may be reduced.

For some applications, it may be desirable to have high phase modulationfrequencies in the range of approximately 300-500 Mhz. This would bedesirable, for example, for applications employing high scanning speedswith high spatial resolution and high reflectance accuracy. In thesecases it may be necessary to abandon piezoelectric fiber stretcher phasemodulators in favor of phase modulators based on electrooptic crystalssuch as lithium niobate. Electrooptic crystal phase modulators aregenerally capable of much higher modulation frequencies.

An alternative is to use an acousto-optic modulator. This device has theadvantage that it gives single sideband modulation which makes theproblem of demodulation of the output signal as a function of the focalpoint location a little simpler if a narrow band laser is used as thesource. With a narrow band, long coherence length laser, the outputsignal from the detectors will vary rapidly in sinusoidal fashion withfocal point location. Since only the envelope of this signal is needed,the envelope must be extracted from this signal by using processes wellknown to those skilled in the art. However, with single sidebandmodulation, the envelope required is obtained as the direct output fromthe detectors. It will be apparent to one skilled in the art of phasemodulator design how to select a specific phase modulator for a givenapparatus and application.

It is noted that the phase modulator can be driven by waveforms otherthan sinewaves. A triangular waveform, for example, creates a morecomplicated pattern of sidebands in the frequency spectrum of the probebeam. These sidebands cause interference in the same manner as asinusoidal driving function, although the resultant heterodyneinterference beats picked up at the detector 70 will be morecomplicated. Also, the phase modulator 54 can be operated in a constantdisplacement mode. For example, a ramp waveform applied to the phasemodulator results in a constant Doppler shift in the frequency of theprobe beam during the sloping part of the ramp waveform. Data can becollected during the sloping part of the ramp and the resultant beatfrequency will be equal to the Doppler shift of the probe beam 44. Itwill be apparent to one skilled in the art of heterodyne interferometrytechniques that there exist many ways of modulating or shifting thefrequency of the probe beam (or reference beam 42 or reflected probebeam 62) such that a detectable beat frequency is generated at thedetectors 70.

The aforementioned embodiments of the present invention inherently lose50% of each of the reference beam 42 and reflected probe beam 62. Inother words, 50% of each beam (reference beam and reflected probe beam)does not contribute to the coherent interference at the detectors 70.More specifically, the portion of the reference beam 42 which travelsdown the short arm 64A of the interferometer does not contribute to theinterference detected by the detectors because it is incoherent withrespect to the reflected probe beam 62. Similarly, the portion of thereflected probe beam 62 which travels down the long arm 64B of theinterferometer does not contribute to interference because it isincoherent with respect to the reference beam. While the first andsecond embodiments are an improvement over the prior art, still furtherimprovements in light usage efficiency can be achieved. This problem isaddressed in the following embodiments of the present invention.

The embodiment of FIG. 10 causes nearly 100% of both the reference beamand reflected probe beam to contribute to the temporal interference atthe detector 70. This is quite beneficial because it greatly increasesthe signal to noise ratio, which in turn makes higher contrast anddeeper images possible.

The embodiment of FIG. 10 uses polarized light techniques toindependently control the reference and reflected probe beams andthereby cause 100% of the reflected probe beam to contribute to thetemporal interference. This embodiment preferably uses polarizationpreserving optical fiber, but bulk optical components may also be used.

The embodiment of FIG. 10 has a fiber coupled polarizing beamsplitter112, and a 90° double pass polarization rotation element 111 located inthe light path. The polarization rotation element can be a ¼ wave plateor faraday rotator. The light path is made of a polarization maintainingfiber 115 which is capable of supporting two orthogonal polarizations.The short arm 64A has a single pass polarization rotator 114 such as a ½wave plate, optically active chiral material, 90° fiber twist, orfaraday rotator. Preferably, both arms 64A, 64B are comprised of lengthsof polarization maintaining optical fiber 115.

A polarized light source emits counterpropagating reference and probebeams which have the same polarization. Both reference and probe beamshave a “lateral” polarization which is represented by arrows 110. Thedouble pass 90° polarization rotation element 111 is located between thelight source and sample 52 so that the reflected probe beam has apolarization orthogonal to the probe beam and reference beam. Concentriccircles 116 represent the polarization of the reflected probe beam.

The reflected probe beam 62 is reflected out of the light path and intothe short arm 64A by the polarizing beamsplitter 112. The reflectedprobe beam then passes through the single pass 90° polarization rotator114 and emerges with a polarization 118 parallel with the referencebeam. Meanwhile, the reference beam propagates through a predeterminedlength of polarization maintaining fiber 115 which comprises the longarm 64B of the interferometer. The reference beam and reflected probebeam are then combined at a directional coupler 82 such that coherentinterference is produced.

The coupler 82 may be a polarization maintaining evanescent wave coupleror fused fiber coupler, for example. The temporal interference caused bythe phase modulator 54 is thus detected and measured and all the lightfrom the reflected probe beam contributes to the interference. Anadjustable attenuator 76 can also be used to attenuate the referencebeam such that maximum signal-to-noise ratio is provided at the detector70. Alternatively, a balanced detection scheme may be used to subtractone component of noise riding on the reference beam. The single pass 90°polarization rotator 114 which effects the reflected probe beam canalternatively be located in the path (long arm 64B) of the referencebeam.

As in the previously described embodiments, the optical path lengthwhich the reference beam travels is selected to match the total pathlength traversed by the reflected probe beam such that coherentinterference occurs at the coupler 82 and detectors 70. Also, a variableoptical delay device can be included anywhere in the system betweencoupler 82 and sample 52 so that small path length adjustments can bemade to maintain coherence between the reference beam and reflectedprobe beam. Adjusting the relative path length allows the coherence gateinterval to be moved longitudinally to the desired fixed position aboutthe focal point 60.

An optical amplifier such as a two-port fiber amplifier may be includedto amplify the reflected probe beam 62. Such an amplifier may be placedanywhere between the polarizing beamsplitter 112 and coupler 82. Thesignal strength of the reflected probe beam is increased if thereflected probe beam is amplified, allowing faster scanning ratesbecause a shorter measurement time is then required for each pixel.

The embodiment of FIG. 10 uses the reflected probe beam much moreefficiently than prior art devices because all of the reflected probebeam contributes to the interference. The signal amplitude is maximizedfor a given intensity of the reflected probe beam, and an improvedsignal-to-noise ratio results.

The phase modulator 54 can be placed in locations other than thelocation shown. For example, it can be placed in the reference beam path(i.e. in the interferometer long arm 64B) so that it modulates only thereference beam. Alternatively, the phase modulator 54 can be placedbetween the polarizing beamsplitter 112 and coupler 82 so that itmodulates only the reflected probe beam.

Placing the phase modulator 54 in one of the interferometer arms 64A,64B may improve the operation of the device because light will only passthrough the phase modulator once. By comparison, if the phase modulatoris located in the light path 50, then the reflected probe beam will havepassed through the phase modulator twice upon its arrival at detector70. Having the modulated light make only a single pass through themodulator 54 is desirable because it avoids possible multiple phasemodulation problems which may arise from the time delay between firstand second passes through the modulator. Single pass modulation,therefore, can increase the maximum possible modulation frequency,thereby improving the resolution of the reflectometer. Such issues willbe apparent to one skilled in the art of heterodyne interferometry.

Some kinds of phase modulators (LiNbO₃ waveguide modulators, forexample) are only transparent to light having a specific polarization. Amodulator of this type cannot be placed in the light path 50, becausethe light path must be transparent to two orthogonal polarizations.Therefore, a modulator of this type must be placed within one of theinterferometer arms 64A, 64B.

An alternative embodiment which uses polarized light is shown in FIG.11. As in the embodiment of FIG. 10, 100% of the reflected probe beamcontributes to the interference, i.e. none of the reflected probe beamis wasted. Here, the interferometer is made of a single length ofbirefringent optical fiber 120. The birefringent fiber performs thefunction of both arms 64A, 64B. No beamsplitters are used, although the90° double pass polarization rotation element 111 is used in the lightpath. The light source of FIG. 11 should be polarized but alsotransparent to an orthogonal polarization. The polarized light sourcemay be a laser with a Brewster window, for example.

The polarization of the reflected probe beam is rotated by 90° withrespect to the original probe beam by the polarization rotation element111. Arrow 110 and concentric circle 116 represent the polarizations ofthe probe beam and reflected probe beam, respectively. The reflectedprobe beam then passes through the polarized light source. Of course,the polarizing element of the light source must not block the reflectedprobe beam. The reflected probe beam then emerges from the referenceaperture 49 combined with the reference beam.

The combined beams are then passed to the length of birefringent opticalfiber 120. The birefringent fiber 120 is oriented such that thereference beam is affected by a higher index of refraction than thereflected probe beam. Therefore, the birefringent fiber 120 providesboth arms 64A, 64B of the interferometer. The length of the birefringentfiber can be selected such that the reference and reflected probe beamsemerge with coherence restored. The beams still have orthogonalpolarizations when they exit the fiber, so an optical mixer 122 isrequired to align (homogenize) the orthogonal polarizations to obtaininterference.

An example of an optical mixer 122 which can be used in the apparatus ofFIG. 11 is shown in FIG. 12. A polarizing beam splitter 123 separatesthe beams by polarization and one beam passes through a ½ wave plate 124or other 90° polarization rotator. Mirrors 126 direct the two beams to abeam splitter 128 where they are combined. The two exiting beams 130then pass into two detectors. It will be apparent to one skilled in theart of optics that other optical mixers 122 can be designed which willwork in the present invention. For example, fiber components such aspolarizing beamsplitters and polarization maintaining evanescent wavecouplers can be used to make fiber-based optical mixers.

It is noted that some kinds of birefringent optical fiber have anon-zero temperature coefficient of birefringence. Therefore, in theembodiment of FIG. 11 the optical path difference for the reference andreflected probe beams 120 can be controlled by adjusting the temperatureof the fiber. The birefringent fiber 120 of FIG. 11 can be placed in atemperature controlled oven 132 to control the path length difference.As a specific example, a 100 meter length coil of high birefringenceoptical fiber can give an optical path displacement of about 10 micronsper ° C. This embodiment provides a method of controlling the opticalpath length and location of the coherence gate interval without movingparts.

Alternatively, control of the optical path length difference between thetwo orthogonal polarization modes in the birefringent fiber 120 can beaccomplished by winding the birefringent fiber around a piezoelectricdrum fiber stretcher. This allows fast adjustment of the position of thecoherence gate interval 78 relative to the focal point 60, as shown inFIG. 3.

FIG. 13 shows an apparatus according to another embodiment of thepresent invention which exploits polarized light. This embodiment has afiber coupled polarizing beamsplitter 112 located between the referenceaperture 49 and the interferometer. Polarization maintaining opticalfiber 115 is used throughout the apparatus. The reflected probe beampasses through the light source 40 and emerges from the referenceaperture 49 combined with the reference beam. The polarizingbeamsplitter 112 then separates the reflected probe beam and referencebeam such that the reflected probe beam is sent through the short arm64A and the reference beam is sent through the long arm 64B.

The short arm 64A includes a 90° single pass polarization rotator 114.The long arm 64B may include the adjustable attenuator 76, as shown.Alternatively, the attenuator may not be used and the method of balanceddetection employed. The coupler 82 is used to recombine the referencebeam and reflected probe beam before they enter the detectors. Thecoupler 82 should be a polarization maintaining coupler. The 90° rotator114 can alternatively be located in the long arm 64B.

FIG. 14 shows an embodiment of the present invention in which both thereference beam and the probe beam are emitted from a single light sourceaperture 133. The single output light source 68 of this embodiment ispolarized and polarization maintaining fiber 115 provides the light pathand the arms 64A, 643 of the interferometer. A first beamsplitter 134(nonpolarizing) is used to create the reference beam from the probe beamemitted from the single aperture 133. The splitting ratio of the firstbeamsplitter 134 in this embodiment can be used to control the power ofthe reference beam 42 such that the signal-to-noise ratio is maximized.This may allow signal-to-noise improvement without the use of theseparate attenuator 76 used in the previous embodiments. A secondpolarizing beamsplitter 112 is located in the light path between thesource and sample.

The nonpolarizing first beamsplitter 134 is oriented such that thereference beam is routed into the long arm 64B. The polarization of thereference beam is indicated by an arrow 150. The probe beam continuesthrough the first beamsplitter 134 to the sample 52. The reflected probebeam returns from the sample and has a polarization orthogonal to theprobe beam and reference beam due to the 90° double pass polarizationrotation element 111. The polarizations of the probe beam and thereflected probe beam are indicated by the arrow 110 and circle 116,respectively.

The reflected probe beam is routed into the short arm 64A by the secondpolarizing beamsplitter 112. The second polarizing beamsplitter 112reflects only the reflected probe beam and not the original probe beambecause the reflected probe beam has a polarization orthogonal to theoriginal probe beam. The polarization rotator 114 is located in theshort arm to align the polarization of the reflected probe beam parallelwith the polarization of the reference beam. The reflected probe beamand reference beam are then combined at a coupler 82 and the resultinginterference is detected at the detectors 70. In the embodiment of FIG.14, all of the reflected probe beam contributes to the interference.

A variable optical delay should be included in the long or short arm ofthe interferometer to provide control over the location of the coherencegate interval 78 with respect to the focal point 60, as shown in FIG. 3.Additionally, an amplifier (such as an optical fiber amplifier) may beincluded in the short arm 64A to amplify the reflected probe beam and toincrease the signal strength of the reflected probe beam. The singlepass polarization rotator 114 can be alternatively located in the longarm 64B. In addition, the phase modulator 54 can be alternativelylocated in the short arm 64A or the long arm 64B.

FIG. 15 shows an apparatus for performing optical coherence domainreflectometry according to another embodiment of the invention. A firstpolarizing beamsplitter 140 is located in the light path between thesingle output light source 68 and the sample 52. The second polarizingbeamsplitter 112 is also located in the light path. The light source inthe embodiment of FIG. 15 may or may not be polarized. Polarizationmaintaining optical fiber 115 is preferably used throughout the system.Light emitted from the single aperture 133 is incident upon the firstpolarizing beamsplitter 140, creating the reference beam which is routedinto the long arm 64B. The probe beam continues through the firstpolarizing beamsplitter 140. The reference beam and the probe beam thushave orthogonal polarizations. Circles 160 represent the polarization ofthe reference beam and arrows 110 represent the polarization of theprobe beam. Circles 160 and arrows 110 indicate orthogonalpolarizations. The reflected probe beam has a polarization parallel withthe reference beam due to the double pass polarization rotation element111. Circles 116 indicates the polarization of the reflected probe beam.The second polarizing beamsplitter 112 is oriented such that thereflected probe beam is routed into the short arm 64A.

The reflectometer of FIG. 15 does not require the polarization rotator114 of FIG. 14. This is an advantage over the embodiment of FIG. 14because it reduces the cost of the device and the number of components.Another advantage of the reflectometer of FIG. 15 is that, in the caseof a polarized light source, the orientation of the first beamsplitter140 determines the amount of optical power which is in the referencebeam. This allows for continuous adjustment of the reference beam powerwithout the need for an optical attenuator in the long arm 64B.

FIG. 15 shows the phase modulator 54 located in the short arm 64A, whichresults in a single pass of the reflected probe beam through themodulator 54. Alternatively, the phase modulator can be located in thelong arm 64B, or anywhere in the light path. Also, a variable opticalpath length delay device may be located in one of the arms 64A, 64B orin the light path for adjusting the location of the coherence gateinterval with respect to the focal point. An amplifier (fiber orsemiconductor) can be placed in arm 64A to boost the strength of thereflected probe beam.

FIG. 16 shows an apparatus according to the preferred embodiment of thepresent invention. The apparatus includes a single polarizingbeamsplitter 140 which is located in the light path between the lightsource 68 and the sample 52. The light source 68 may be polarized ornonpolarized. Polarization maintaining optical fiber 115 is preferablyused throughout the apparatus. Light emitted by the single aperture 133is incident upon the polarizing beamsplitter 140, creating the referencebeam and the probe beam.

The reference beam is directed by the beamsplitter 140 into the long arm64B and has a polarization represented by the circles 160. The probebeam has a polarization represented by the arrows 110. The reflectedprobe beam has a polarization parallel with the reference beam due tothe double pass polarization rotation element 111. The reflected probebeam has a polarization represented by the circles 116. The reflectedprobe beam is directed into the short arm 64A by the polarizingbeamsplitter 140. The reference beam and reflected probe beam, havingparallel polarizations, are combined at the coupler 82.

There are many types of fiber couplers that may be used in theembodiments of FIGS. 6, 10, 11, 13, 14, 15, and 16. For example, thefunction of first nonpolarizing beamsplitter 134 can be realized using apolarization maintaining variable ratio evanescent wave coupler. Thefunction of polarizing beamsplitter 112 can be realized using apolarizing beamsplitter evanescent wave coupler, and coupler 82 can berealized using a polarization maintaining 50/50 fixed ratio evanescentwave coupler. The use of these fiber couplers is well known in the artof fiber-optic gyroscopes and fiber interferometers.

The scanning heads of FIGS. 8A, 8B, or 8C can be combined with theembodiments of FIGS. 2, 6, 10, 11, 13, 14, 15, or 16 to allow forimaging. It will be apparent to one skilled in the art of constructingoptical systems how to combine these elements.

The present invention provides a reflectometer which can producereal-time video images of the internal structure of turbid materials.Thus, the present invention can be used to provide images of biologicaltissues. For example, the reflectometer of the present invention can beused to image through skin or within arteries or vessels to analyze thearterial walls. For imaging through biological tissue or human skin,certain spectral regions are known which have low absorption andscattering coefficients and therefore allow deeper imaging or betterimage contrast. For example, it is well known that human skin isparticularly transparent in the wavelength range of 0.8 to 1.6 microns.Therefore, a light source which produces light in this range should beused in the apparatus of the present invention when attempting to imagethrough human skin. It will be apparent to one skilled in the art ofimaging that such considerations are important in choosing the lightsource and wavelength range for a particular application.

The invention can be made using a small number of fiber components forreliability, low cost, flexibility, and ease of assembly.

It will be clear to one skilled in the art that the above embodimentsmay be altered in many ways without departing from the scope of theinvention. Accordingly, the scope of the invention should be determinedby the following claims and their legal equivalents.

What is claimed is:
 1. An apparatus for performing optical coherencedomain reflectometry on a sample, the apparatus comprising: a) a twooutput polarized light source having: i) a probe aperture for emitting aprobe beam; and ii) a reference aperture for emitting a reference beam;wherein the light source is located such that a light path existsbetween the probe aperture and the sample. b) a 90° double passpolarization rotation element located in the light path such that areflected beam reflected from the sample is orthogonally polarized withrespect to the probe beam; c) an interferometer having a short arm and along arm, wherein the long arm is positioned to receive the referencebeam; d) a polarizing beamsplitter located in the light path fordiverting the reflected beam into the short arm, wherein the long armand the short arm have an optical path length difference selected toprovide optical coherence between the reference beam and the reflectedbeam; and e) a polarization rotator located in the interferometer suchthat the reference beam and the reflected beam have substantially thesame polarization at the output of the interferometer.
 2. The apparatusof claim 1 further comprising a transverse scanning mechanism in thelight path for producing reflectance images.
 3. The apparatus of claim 2wherein the scanning mechanism comprises at least one scanning mirror.4. The apparatus of claim 3 wherein the scanning mirror is amicromachined scanning mirror.
 5. The apparatus of claim 3 wherein thescanning mirror scans about one axis.
 6. The apparatus of claim 3wherein the scanning mirror scans about two axes.
 7. The apparatus ofclaim 1 further comprising a vertical scanning mechanism in the lightpath for scanning in a direction parallel with the probe beam.
 8. Theapparatus of claim 1 wherein the light path further comprises a spatialfilter for providing spatial filtering of the reflected beam.
 9. Theapparatus of claim 1 wherein the light path comprises a lens assemblyfor focusing the probe beam to a point within the sample, the lensassembly having a numerical aperture in the range of 0.4 to 1.4.
 10. Theapparatus of claim 1 wherein the interferometer comprises twopolarization maintaining optical fibers of unequal length.
 11. Theapparatus of claim 1 wherein the light path comprises a polarizationmaintaining optical fiber capable of supporting two independentorthogonal polarization modes.
 12. The apparatus of claim 1 furthercomprising a phase modulator for modulating the phase of either thereference beam or the reflected beam.
 13. The apparatus of claim 12wherein the phase modulator is located within the light path.
 14. Theapparatus of claim 12 wherein the phase modulator is located within onearm of the interferometer.
 15. The apparatus of claim 12 wherein thephase modulator is a piezoelectric fiber stretcher, an electroopticcrystal, or an acousto-optic modulator.
 16. The apparatus of claim 1further comprising a frequency shifting means for shifting the frequencyof either the reference beam or the reflected beam.
 17. The apparatus ofclaim 16 wherein the frequency shifting means is located within thelight path.
 18. The apparatus of claim 16 wherein the frequency shiftingmeans is located within one arm of the interferometer.
 19. The apparatusof claim 1 wherein the polarizing beamsplitter is a polarizingbeamsplitter evanescent wave optical fiber coupler.
 20. The apparatus ofclaim 1 wherein the double pass 90° polarization rotation elementcomprises a Faraday rotator.
 21. The apparatus of claim 1 furthercomprising a variable optical delay device in at least one arm of theinterferometer.
 22. The apparatus of claim 1 further comprising avariable optical delay device disposed in the light path.
 23. Theapparatus of claim 1 wherein the coherence length of the light source isless than 3000 microns.
 24. The apparatus of claim 1 wherein the shortarm comprises an optical amplifier for amplifying the reflected beam.25. The apparatus of claim 1 wherein the light source is of the typewhich produces light in the wavelength range of 0.8 to 1.6 microns. 26.The apparatus of claim 1 further comprising an optical detector.
 27. Amethod for performing optical coherence domain reflectometry on asample, the method comprising the steps of: a) producing a referencebeam and a probe beam from a dual output polarized light source having areference aperture and a probe aperture, wherein the reference beam andthe probe beam are counterpropagating, and wherein the reference beamemerges from the reference aperture and the probe beam emerges from theprobe aperture; b) passing the probe beam into the sample through apolarizing beamsplitter, wherein a portion of the probe beam isreflected by the sample to form a reflected beam; c) rotating thepolarization of the reflected beam compared to the probe beam such thatthe reflected beam and the probe beam have orthogonal polarizations. d)routing the reflected beam into a short arm of an interferometer usingthe polarizing beamsplitter; e) sending the reference beam to a long armof the interferometer; f) rotating the polarization of either thereflected beam or the reference beam such that the reflected beam andthe reference beam have substantially the same polarization; and g)combining the reference beam and the reflected beam such that coherentinterference is produced.
 28. The method of claim 27 further comprisingthe step of modulating the phase of either the reference beam or thereflected beam to modulate the coherent interference.
 29. The method ofclaim 27 further comprising the step of shifting the frequency of eitherthe reference beam or the reflected beam to modulate the coherentinterference.
 30. The method of claim 27 further comprising the step ofspatially filtering the reflected beam.
 31. The method of claim 27further comprising the step of optically amplifying the reflected beamin the short arm.
 32. The method of claim 27 further comprising the stepof adjusting an optical path length of at least one arm of theinterferometer.
 33. The method of claim 27 further comprising the stepof optically detecting the coherent interference.